Cheat mode:
To unlock a cheats menu that will display in the options menu during game play, complete the game.

Zombie mode:
To unlock Zombie mode, pause the game and press up, down, up, down, left, right, left, right.

Intel pieces unlockables:
Collect the following amounts of intel pieces to unlock the corresponding cheat options:

Collect 15 intel pieces - A Bad Year: Enemies explode into tires when shot.
Collect 10 intel pieces - Cluster Bombs: When a frag grenade explodes, four more will explode in a cross-shaped pattern.
Collect 2 intel pieces - CoD Noir: Graphics turn black and white) -
Collect 30 intel pieces - Infinite Ammo: Reloads are no longer necessary. This has no effect with single-shot weapons such as RPGs, C4, or claymores.
Collect 4 intel pieces - Photo-Negative: Inverts the colors.
Collect 8 intel pieces - Ragtime Warfare: Silent movie mode.
Collect 20 intel pieces - Slow-Mo Ability: Slows game to half speed when you melee.
Collect 6 intel pieces - Super Contrast: Increases the contrast.

Intel piece locations:
The corresponding amount of intel pieces are located in the following levels:

Level 2 - 2 intel pieces
Level 4 - 2 intel pieces
Level 5 - 3 intel pieces
Level 6 - 2 intel pieces
Level 7 - 2 intel pieces
Level 9 - 3 intel pieces
Level 10 - 2 intel pieces
Level 12 - 2 intel pieces
Level 13 - 3 intel pieces
Level 14 - 2 intel pieces
Level 16 - 2 intel pieces
Level 17 - 1 intel pieces
Level 18 - 2 intel pieces
Level 19 - 2 intel pieces

Arcade mode:
To unlock Arcade mode, beat the game.

Prestige mode:
To unlock Prestige mode, reach level 55. It is a multiplayer-only mode that starts you again at level 1.

Epilogue mission:
To unlock the Epilogue mission, complete the game and watch the entire credits.

Mile High Club mission:
To unlock the Mile High Club mission, complete story mode under any difficulty setting.

Golden weapons:
Unlock the following Golden Guns by completing the corresponding weapons challenges. Note: This does not boost the guns performance; only the appearance is changed:

Golden Ak-47 - Complete all assault rifle challenges.
Golden Desert Eagle - Reach level 55.
Golden Dragonuv - Complete all sniper challenges.
Golden M1014 - Complete all shotgun challenges.
Golden M60 - Complete all LMG challenges.
Golden Mini-Uzi - Complete all SMG challenges.

Multiplayer weapons:
Reach the following ranks in Multiplayer mode to unlock the corresponding bonuses:

Private
Level 1 - All starting equipment
Level 2 - Demolitions class weapon class
Level 3 - Sniper class weapon class

Lance Corporal
Level 4 - Create a class
Level 5 - Gun challenges
Level 6 - New playlists

Corporal
Level 7 - M21 Sniper rifle
Level 8 - Last stand perk class 3
Level 9 - Boot camp challenges 1

Sergeant
Level 10 - M4 carbine assault rifle
Level 11 - UAV jammer perk class 2
Level 12 - Clan tag

Staff Sergeant
Level 13 - Mini uzi submachine gun
Level 14 - Bomb squad perk class 1
Level 15 - Boot camp challenges 2

Gunnery Sergeant
Level 16 - M1911 pistol
Level 17 - Martyrdom perk class 2
Level 18 - Boot camp challenges 3

Master Sergeant
Level 19 - M60E4 light machine gun
Level 20 - Sleight of hand perk class 2
Level 21 - Operations challenges

Master Gunnery Sergeant
Level 22 - Dragunov sniper rifle
Level 23 - Claymore perk class 1
Level 24 - Operations challenges 2

2nd Lieutenant
Level 25 - G3 assault rifle
Level 26 - Iron lungs perk class 3
Level 27 - Operations challenges 3

1st Lieutenant
Level 28 - AK-74U submachine gun
Level 29 - Double tab perk class 2
Level 30 - Killer challenges

Captain
Level 31 - M1014 shotgun
Level 32 - Bandolier perk class 1
Level 33 - Killer challenges 2

Major
Level 34 - R700 sniper rifle
Level 35 - Eavesdrop perk class 3
Level 36 - Killer challenges 3

Lieutenant Colonel
Level 37 - G36C assault rifle
Level 38 - Overkill perk class 2
Level 39 - Killer challenges 4

Colonel
Level 40 - P90 submachine gun
Level 41 - Frag x3 perk class 1
Level 42 - Humiliation challenges

Brigadier General
Level 43 - Desert eagle pistol
Level 44 - Dead silence perk class 3
Level 45 - Humiliation challenges 2

Major General
Level 46 - M14 assault rifle
Level 47 - Humiliation challenges 3
Level 48 - Humiliation challenges 4

Lieutenant General
Level 49 - Barret sniper rifle
Level 50 - Humiliation challenges 5
Level 51 - Elite challenges

Commander
Level 52 - MP44 assault rifle
Level 53 - Elite challenges 2
Level 54 - Elite challenges 3
Level 55 - Commander prestige mode and golden desert eagle

Multi-player headshots:
A metal "ping" sound will confirm a headshot kill.

Better aiming accuracy:
Use the red dot scope over the ACOG scope. The red dot will give you a better aimpoint on most weapons. The ACOG scope will actually add more range, but take away accuracy.

Level up quicker:
Use the same gun until all the marksman and elite challenges are completed. This will quickly level you up to new guns, but do not use them. Keep using the same one you started with until the challenges are completed.